A multimode mobile terminal, such as a radiotelephone or personal communicator, is capable of operating in more than one system. By example, and for a dual mode mobile terminal, the modes may be a digital cellular mode (e.g., GSM or DCS1900) and an analog cellular mode (e.g., IS-41 (AMPS)).
As can be appreciated, it is important that a multimode mobile terminal be capable of automatically switching between the available operating modes when conditions warrant.
Some types of known networks do provide for inter-network roaming (e.g., DAMPS/AMPS). Reference can be had to a publication entitled "Implementation Guide: Non-Public Mode Operation and Selection in IS-136 Compliant Mobile Stations", Version 2.0, TDMA Forum, Mar. 9, 1995.
It can thus be appreciated that the capability to switch between networks becomes especially important when the terminal operates with networks that do not support inter-network roaming (e.g., DCS1900 and AMPS).
One technique has been proposed in a publication entitled "Inter-Network Roaming Selection", North American Interest Group (NAIG) of the GSM MoU, Jun. 21, 1996 (Ericsson). This publication describes a limited PCS1900/AMPS solution using a systems priority list, wherein a handset moves from the digital network (PCS1900) to the analog network (AMPS). However, this proposal does not provide full, bidirectional mode selection, in that no provision is made to automatically move from the analog network back to the digital network.
Also of interest in this area are U.S. Pat. No. 5,504,803, entitled "Method for Automatic Mode Selection for a Dual-Mode Telephone Handset for use in a Cellular Mobile Telephone System and in a Wireless Telephone System", by Yamada et al., and U.S. Pat. No. 4,916,728, entitled "Cellular Telephone Unit with Prioritized Frequency Acquisition", by Blair.